1. Field of the Invention
The present invention relates to nuclear reactors. More particularly, the present invention relates to a self-powered detector for in-core measurement of local reactor power level and neutron flux density.
2. Description of the Prior Art
Present day fission chambers that are used in the core of a nuclear reactor contain a ceramic seal between the chamber and its integral coaxial cable. The purpose of this seal is to maintain a constant quantity of gas in the fission chamber in order to provide a neutron sensitivity that is independent of the temperatures of the chamber and its integral cable. A majority of these seals develop leaks during the exposure interval 10.sup.21 to 10.sup.22 nvt, after which interval the chamber loses its desirable property of temperature-independent neutron sensitivity. The chamber then suffers from a larger nonlinearity with reactor power level, a change in sensitivity after a cold start-up, and, in the case of bottom-entry detectors, a change in sensitivity after a change in rod bank position or in power level.
Self-powered neutron detectors do not depend upon the gas in the sensitive region for their current output. Accordingly, no seal is required between the detector and its integral cable. Thus, neutron sensitivity is independent of the temperature of the detector and that of its integral cable. Such detectors are useful for measuring local power level or neutron flux density after the reactor has been in a steady state for several minutes, but, because of their slow speed of response, they do not give correct readings immediately after a power level change. They cannot be used to provide a warning or a scram signal if the reactor power level becomes too high, as in-core fission chambers are often called upon to do in water reactors.
For example, after a step change in local neutron flux density, the output of a rhodium self-powered detector changes 61% of the flux density change in one minute, 82% in two minutes, 90% in three minutes, 96% in five minutes, and 98% in ten minutes. It is readily apparent the detector-based reading can be in serious error for the first five or ten minutes of operation. Such dilatory response is undesirable in a nuclear reactor.
Pertinent prior art publications include: U.S. Pat. No. 2,874,304, entitled "Ionization Chamber", issued to Lichtenstein on Feb. 17, 1959; U.S. Pat. No. 3,043,954, entitled "Fission Chamber Assembly", issued to Boyd et al. on July 10, 1982; U.S. Pat. No. 3,565,760, entitled "Nuclear Reactor Power Monitor System", issued to Parkos et al. on Feb. 23, 1971; U.S. Pat. No., 4,103,166, entitled "Method and Apparatus for Monitoring the Output of a Neutron Detector", issued to Neissel et al. on July 25, 1978; U.S. Pat. No. 3,760,183, entitled "Neutron Detector System", issued to Neissel on Sept. 18, 1973; and the publications Dynamic Compensation of Rhodium Self-Powered Neutron Detectors, Banda et al., IEEE Transactions on Nuclear Science, Vol. NS-23, No. 1 (February 1976), and A Pena-Detector Flux-Measure System With A Fast Response, Johnstone, Research Reactors Div., U.K.A.E.A. Research Group, Atomic Energy Research Establishment (March 1971). So far as any of the above-mentioned patents might be considered necessary, either in whole or in part, to assist or enable one skilled in the art to practice the herein disclosed invention, they are hereby incorporated into this patent application by reference.